Non-volatile memory cells are commonly used to store data derived from an associated volatile memory cell so that the data is not lost during a power-off period. These non-volatile cells are commonly referred to as "shadow" cells. The connections between the non-volatile cell and the volatile cell are usually bi-directional; they function as inputs during a non-volatile store operation and as outputs during a recall of the data to the volatile cell from the non-volatile cell. A source of relatively high voltage, on the order of from 10 to 20 volts, induces tunneling in one of a pair of Fowler-Nordheim (FN) isolation elements in the non-volatile cell during store operations. The FN element has a very high impedance when not in a tunneling state so that the stored charge is isolated from the rest of the circuit during power-off.
U.S. Pat. No. 4,510,584 issued to Leuschner, Guterman, Proebsting and Dias for a non-volatile memory cell is illustrated in the prior art schematic diagram of FIG. 1. Two Fowler-Nordheim (FN) tunneling elements 20, 22 are employed to store and isolate a charge on the gate of Q.sub.24. Capacitor C.sub.26 completes the isolation circuit. C.sub.26 may be a FET type device. FN devices, 20, 22 may be implemented as two conducting layers separated by a thin (100 Angstrom) dielectric layer, such as SiO.sub.2. Due to the bending of the conduction bands in the conductor layers, it is possible for electrons to tunnel through the dielectric layer with increasing probability as the electric field across the dielectric layer increases. The Fowler-Nordheim equation for this behavior is: EQU J=a*E.sup.2 *e.sup.(-b/E)
where:
J=current density (A/cm.sup.2) PA1 E=electric field strength (V/cm) PA1 a,b=constants PA1 e=2.718 . . .
For the purpose of explanation of the circuits to be described, infra, the FN element may be considered as a bidirectional voltage triggered switch, similar to a pair of Zener diodes connected back to back. If the absolute potential across the FN element is less than the Fowler-Nordheim voltage the element is non-conducting and if the potential exceeds the Fowler-Nordheim voltage, then the element conducts with a very low impedance.
The advantage of the cell of FIG. 1 lies in the fact that it may be easily connected to a volatile storage element (not shown) via nodes D, D and that such connection does not interfere with normal operation of the volatile element. Thus, it is possible and practical to achieve both high speed operation and non-volatility. Recall of the non-volatile data occurs via node 21 which is usually connected to the volatile memory cell which provided the D and D inputs. With single controlling transistor Q.sub.24 and floating node 32 it is necessary to provide a minimum voltage difference between the two complementary data states of at least two to three volts because sensing transistor Q.sub.24 must be turned on strongly enough in one data state to overcome the volatile element should it be in the opposite state. The voltage difference between the two complementary data states of the non-volatile element is directly related to the amount of charge passed through FN tunneling elements 20, 22.
The reliability of tunneling elements 20, 22, is, in turn, directly related to the oxide charge transfer and it is desirable to minimize the tunneling charge transfer to achieve maximum reliability. In addition, reliability is significantly dependent upon the oxide electric field/current density and therefore, another objective is to minimize oxide current density.
These requirements are believed to have led to the circuit of FIG. 2 which has been marketed as a portion of Mostek Part No. MK 4701 where it is used as a reference cell. It may be observed, by comparing FIGS. 1 and 2, that the cell of FIG. 1 appears, in large part, in two places in FIG. 2. (Identical reference numerals are used in each half-module of FIG. 2 and the A and B suffices indicate a part which is used in both modules.) They are connected in complementary fashion to the volatile storage element (again, not shown) via nodes D and D, as before. The important difference is that in the circuit of FIG. 2, nodes D and D serve both as input and output nodes. Because of the complementary nature of the circuit of FIG. 2, and the symmetry of the storage cell, it is not necessary to achieve a large voltage difference in the two data states stored simultaneously on nodes 32A and 32B, as it was in the circuit of FIG. 1. Thus it is possible to reduce tunneling current requirements and to enhance the reliability of FN tunneling elements 20A, 22A, 20B and 22B.
However, the prior art circuits of FIGS. 1 and 2 employ two tunnel devices per cell (in the case of FIG. 1) and four tunnel devices per cell (in the case of FIG. 2). Since the yield and reliability of such cells are a strong function of the number of tunneling devices used in each such cell, it would be advantageous to reduce the number of tunnel devices per cell.
In the circuits of FIGS. 1 and 2, FN devices 20, 22 always form a series path between the high voltage source and ground. This means that the high voltage must be chosen so that it never exceeds twice the Fowler-Nordheim voltage of the tunneling elements since, under that condition, both devices would conduct and a large current would flow from the high voltage node to the ground node. Such a large current would impact the reliability of the FN devices severely. It is clear that if either FN device of the pair were to fail, the circuit would disfunction.
In the circuits of the prior art, current always flows through a given FN device in the same direction. This is known to give rise to a phenomenon, well known in the art, as Fowler-Nordheim voltage "walkout," which has been shown to be a precursor to device breakdown and failure of the thin tunneling medium.